Field emission cathodes are electron emitting devices which are used, for example, in flat panel displays. A field emission cathode or "field emitter" emits electrons when subjected to an electric field of sufficient strength. A side sectional view depicting conventional steps used to manufacture a field emission cathode is shown in Prior Art FIG. 1A. More specifically, in Prior Art FIG. 1A, a first conductive layer or "row electrode" 102 has a resistive layer 104 disposed thereon. An inter-metal dielectric layer 106 disposed above resistive layer 104 has a cavity 108 formed therein. As shown in Prior Art FIG. 1A, a second conductive layer or gate electrode 110 resides above inter-metal dielectric layer 106. A hole or opening 112 is formed through gate electrode 110 directly above cavity 108. Opening 112 is used to form the field emitter which will reside within cavity 108. Typically, the formation of the field emitter is accomplished, in part, using a lift-off or "parting layer", and a closure layer.
With reference next to Prior Art FIG. 1B, a side sectional view illustrating the deposition of a lift-off layer 114 is shown. Lift-off layer 114 is commonly formed using an angled physical vapor deposition of, for example, aluminum. Arrows 118 illustrate the angled nature of the deposition of lift-off layer 114. The angled deposition of lift-off layer 114 is required to insure that no lift-off layer material, i.e. aluminum, is deposited into the bottom of cavity 108. In order to achieve an angled deposition, the entire field emitter structure must be rotated during the deposition of lift-off layer 114.
Referring next to Prior Art FIG. 1C, a side sectional view illustrating the initial formation of a closure layer 118 is shown. Closure layer 118 is comprised of electron emissive material such as, for example, molybdenum. The electron emissive material which forms closure layer 118 is also deposited into cavity 108 as shown by structure 120. Typically, the electron emissive material is deposited using, for example, an e-beam evaporative deposition method.
Referring now to Prior Art FIG. 1D, a side sectional view illustrating a completed deposition of electron emissive material is shown. As shown in Prior Art FIG. 1D, closure layer 118 completely seals cavity 108. Additionally, as the electron emissive material is deposited as shown in Prior Art FIGS. 1C and 1D, an electron emitting structure 120 commonly referred to as a "Spindt-type" emitter is formed within cavity 108 (Spindt-type emitters are described in detail in U.S. Pat. No. 3,665,241 to Spindt et al. which is incorporated herein by reference as background material). After emitter 120 is formed, closure layer 118 must be removed.
With reference now to Prior Art FIG. 1E, a side sectional view illustrating the removal of closure layer 118 is shown. When removing closure layer 118, care must be taken not to damage or otherwise adversely affect emitter 120. Such a removal process is further complicated by the fact that both closure layer 118 and emitter 120 are formed of the same electron emissive material. Prior art techniques remove closure layer 118 by etching lift-off layer 114 using an etchant which attacks the aluminum lift-off layer 114. As a result, lift-off layer 114 "lifts" from underlying gate electrode 110 and, consequently, removes closure layer 118, as illustrated in Prior Art FIG. 1E.
Unfortunately, such prior art lift-off and closure layer removal methods typically expose the field emitter structure to the etchant for an extended period of time. Specifically, some prior art lift-off layer and closure layer removal processes expose the field emitter structure to an etchant for as long as hours. Such extended exposure to the etchant results in damage to the emitters. Such prior art lift-off and closure layer removal processes also result in the generation of flakes or contaminating chunks, typically shown as 122a-122d, which contaminate the etchant. Flakes or chunks 122a-122d can also redeposit within or over cavity 108, as shown by chunk 122c, and compromise the integrity of emitter 120 formed therein. As a result, the emitter can be severely damaged or even shorted to gate electrode 110, or otherwise affect emission.
Conventional lift-off and closure layer removal methods are not always entirely effective. That is, additional subsequent process steps may be necessary to insure that the lift-off and closure layer are completely removed. As an example, some prior art methods require that the lift-off and closure layer be physically rubbed from the gate electrode even after prolonged exposure to the etchant. Other prior art methods apply a tape to the closure layer after exposure to the etchant. The tape adheres to those portion of the lift-off and closure layers which remain on the gate electrode. The remaining portions of the lift-off and closure layers are then removed by peeling the tape from the field emitter structure. Such post-etch lift-off and closure layer removal process are extremely time-consuming, labor-intensive, and are not well suited for high volume production.
As yet another drawback, conventional lift-off and closure layer removal processes are not well suited for use with field emitter structures containing focusing walls. That is, prolonged exposure to various prior art etchants can deteriorate the focus walls. Also, in prior art approaches, the focus walls can prevent portions of lifted or detached lift-off and closure layers from migrating away from the gate electrode. As a result, the lifted lift-off and closure layer will redeposit back onto the gate electrode. Additionally, post-etch processes such as hand-rubbing or tape-peeling of the lift-off and closure layers is further complicated by the presence of focus wall structures.
Thus, a need exists for a lift-off and closure layer removal method which does not require exposing the field emitter structure to etchants for a prolonged period of time. A further need exists for a lift-off and closure layer removal method which does not require subsequent rubbing or tape-peeling processes to completely remove the lift-off and closure layers. Still another need exists for a lift-off and closure layer removal method which is compatible with the use of focus walls.